CI: github actions: use cache for OpenTracing

this caches OpenTracing libs between builds, should save couple of minutes
for each build.

(cherry picked from commit b9e3fb7315e32cee678d3635f715de65b6c53fae)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>
(cherry picked from commit 4de35027d01617fcbd928f5b27132d46d0017cd6)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>
diff --git a/.github/matrix.py b/.github/matrix.py
index 91ba34b..74bfcc7 100644
--- a/.github/matrix.py
+++ b/.github/matrix.py
@@ -72,8 +72,8 @@
             "FLAGS": [
                 "USE_ZLIB=1",
                 "USE_OT=1",
-                "OT_INC=${HOME}/opt/include",
-                "OT_LIB=${HOME}/opt/lib",
+                "OT_INC=${HOME}/opt-ot/include",
+                "OT_LIB=${HOME}/opt-ot/lib",
                 "OT_RUNPATH=1",
                 "USE_PCRE=1",
                 "USE_PCRE_JIT=1",
@@ -147,8 +147,8 @@
         + [
             "USE_ZLIB=1",
             "USE_OT=1",
-            "OT_INC=${HOME}/opt/include",
-            "OT_LIB=${HOME}/opt/lib",
+            "OT_INC=${HOME}/opt-ot/include",
+            "OT_LIB=${HOME}/opt-ot/lib",
             "OT_RUNPATH=1",
             "USE_PCRE=1",
             "USE_PCRE_JIT=1",